JPH054375Y2 - - Google Patents

Description

[Detailed description of the invention] (Field of industrial application) The present invention relates to an FM television broadcast receiving device that is capable of receiving weak FM television broadcast signals such as satellite broadcasting with an excellent S/N ratio. It is something.

(Prior art) In recent years, satellite broadcasting receiving systems have become popular even for home use, and weak satellite broadcasting signals can be transmitted through excellent S/S/
There is a strong demand for a receiving system that can receive at an N ratio. Therefore, attempts have been made to improve the noise figure by using low-noise group conversion circuits and low-noise amplification circuits in antenna devices for receiving satellite broadcasting. Further, noise power is improved by narrowing the bandwidth of the receiving device.

FIG. 3 shows a block circuit diagram of a conventional FM television broadcast receiving apparatus in which the bandwidth is narrowed to improve noise power.

In FIG. 3, a high frequency signal of an FM television signal is given to a frequency conversion circuit 1, mixed with a local oscillation signal input from a local oscillation circuit 2 and frequency converted, and an intermediate frequency signal is extracted by a bandpass filter 3. be done. The signal is then sent via an intermediate frequency amplifier circuit 4 and an amplitude limiting circuit 5 to a demodulation circuit 6 consisting of a frequency discriminator, where it is demodulated into a baseband signal. This demodulated signal is transmitted to the low frequency amplifier circuit 7.
The signal is amplified appropriately and output to the output terminal 8. Also, a part of this demodulated signal is given to the local oscillation circuit 2 as frequency compression negative feedback to change the frequency of the local oscillation signal, and the high frequency signal having the baseband signal as shown in FIG. 4a is changed to the high frequency signal shown in FIG. 4b. The baseband signal with a narrow bandwidth is converted into an intermediate frequency signal such as a narrow bandwidth baseband signal. Therefore, the intermediate frequency signal can pass through the bandpass filter 3 having a narrow bandwidth w as shown by the solid line in FIG. 4c, reducing the noise power of the receiving device and lowering the threshold.

(Problem to be solved by the invention) By the way, in the above embodiment, in the period before one horizontal scan of the baseband signal, the local oscillation signal changes according to the level of the demodulation signal, resulting in frequency compression negative feedback. Therefore, there is a problem that the differential gain, differential phase, etc. of the output signal are likely to deteriorate due to response delay of the local oscillation signal. In addition, in order to improve the response delay of the local oscillation signal, if the loop gain of the negative feedback circuit that changes the local oscillation signal with respect to the demodulated signal is increased, the local oscillation signal may change erroneously due to noise entering the negative feedback circuit. It is easy to deteriorate the output signal.

The purpose of this invention was to solve the above-mentioned problems with conventional FM television broadcast receivers, and it improves the S/N ratio by compressing frequency deviation without deteriorating the video signal. will be done
The purpose of the present invention is to provide an FM television broadcast receiving device.

(Means for Solving the Problems) In order to achieve this purpose, the FM television broadcast receiving device of the present invention includes a frequency conversion circuit,
A bandpass filter that extracts an intermediate frequency signal from the output of the frequency conversion circuit, a demodulation circuit that demodulates the intermediate frequency signal, a synchronization extraction circuit that extracts a synchronization signal from the demodulation signal, and a synchronization extraction circuit that extracts a synchronization signal from the demodulation signal. a local oscillation circuit that compresses the frequency deviation of the intermediate frequency signal by changing the frequency of the local oscillation signal applied to the frequency conversion circuit by the period of the synchronization signal; and adding the extracted synchronization signal to the demodulated signal. It consists of a synchronous mixer and a synchronous mixer.

(Function) A sync signal is extracted from the demodulated signal, and the local oscillator signal is changed only during the period of the sync signal in response to the extracted sync signal, and the frequency shift of only the sync signal portion is compressed to reduce noise power and improve the S/N ratio. Since the sync signal is compressed and added to the demodulated signal to produce an output signal, the video signal portion does not undergo compression processing, and the differential gain and differential phase are not degraded.

(Example) Hereinafter, an example of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a block circuit diagram of an embodiment of the FM television broadcast receiving apparatus of the present invention, and FIG. 2 is a diagram showing the signal waveforms of each part of FIG. 1 and the characteristics of the bandpass filter.
In Figures 1 and 2, Figures 3 and 4
Circuits and the like that are the same as those in the figures are given the same reference numerals and redundant explanations will be omitted.

In FIGS. 1 and 2, a demodulated signal output from demodulation circuit 6 is applied to low frequency amplification circuit 7 and also to synchronization extraction circuit 9. In FIG. The synchronization signal is extracted by the synchronization extraction circuit 9 as shown in FIG. 2d, inverted and amplified by the synchronization signal amplification circuit 10, and then applied to the local oscillation circuit 2 as a signal shown in FIG. 2e. Here, the local oscillation circuit 2 changes the frequency of the local oscillation signal only during the period when the synchronization signal exists, and gives frequency compression negative feedback to the high frequency signal, so that the high frequency signal having the baseband signal as shown in FIG. Only the synchronization signal portion as shown in FIG. 2b is converted into an intermediate frequency signal of the compressed narrow bandwidth baseband signal. Therefore,
As shown by the solid line in FIG. 2c, the intermediate frequency signal can pass through the bandpass filter 3 having a narrow bandwidth W, thereby reducing the noise power. Further, the demodulated signal suitably amplified by the low frequency amplifier circuit 7 is sent to a synchronous mixer 11.
is added to the synchronization signal supplied from the synchronization signal amplification circuit 10, and as shown in FIG. 8 is output.

(Effects of the invention) As explained above, according to the FM television broadcast receiving device of the invention, by compressing the frequency deviation of the synchronization signal part, the intermediate frequency signal is narrowed in bandwidth and the noise power is improved. . Moreover, the video signal part is not subjected to any compression processing, so there is no deterioration of phase gain or phase differential, and weak signal processing is not performed.
It has the excellent effect of being able to receive FM television broadcast signals with a good S/N ratio.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram of an embodiment of the FM television broadcast receiving apparatus of the present invention, and FIG. 2 is a diagram showing the signal waveforms and bandpass filter characteristics of each part of FIG. 1. Figure 3 shows the conventional
FIG. 4 is a block circuit diagram of an example of an FM television broadcast receiving apparatus, and FIG. 4 is a diagram showing signal waveforms and bandpass filter characteristics of each part in FIG. 3. 1... Frequency conversion circuit, 2... Local oscillation circuit,
3... Bandpass filter, 6... Demodulation circuit, 9
... Synchronous extraction circuit, 11... Synchronous mixer.

Claims (1)

[Scope of utility model registration request] a frequency conversion circuit; a bandpass filter for extracting an intermediate frequency signal from the output of the frequency conversion circuit; a demodulation circuit for demodulating the intermediate frequency signal; a synchronization extraction circuit for extracting a synchronization signal from the demodulation signal; a local oscillation circuit that compresses the frequency deviation of the intermediate frequency signal by changing the frequency of a local oscillation signal given to the frequency conversion circuit according to a synchronization signal for a period of the synchronization signal; An FM television broadcast receiving device consisting of a synchronous mixer that adds synchronous signals.